Custom Exceptions

Learn how to define your own exception types by inheriting from the Exception class.

Why Custom Exceptions?

Built-in exceptions like `ArgumentException` are general-purpose. A custom exception type lets you represent domain-specific failures clearly, and lets calling code catch exactly the failure it cares about.

Creating a Custom Exception

class InsufficientFundsException : Exception
{
    public decimal Shortfall { get; }

    public InsufficientFundsException(decimal shortfall)
        : base($"Insufficient funds. Short by {shortfall:C}.")
    {
        Shortfall = shortfall;
    }
}

Throwing and Catching It

class Account
{
    public decimal Balance = 100;

    public void Withdraw(decimal amount)
    {
        if (amount > Balance)
            throw new InsufficientFundsException(amount - Balance);

        Balance -= amount;
    }
}

var account = new Account();
try
{
    account.Withdraw(150);
}
catch (InsufficientFundsException ex)
{
    Console.WriteLine(ex.Message);
    Console.WriteLine($"Shortfall: {ex.Shortfall:C}");
}

Output

Click Run to see what this code prints.

FAQs

Should every possible error get its own exception class?

No — reserve custom exceptions for meaningful, domain-specific failures your calling code will actually want to handle differently. Overusing them adds noise.

Summary

Custom exceptions inherit from Exception (or a more specific built-in type).
They can carry extra data relevant to the failure.
Use them for meaningful, domain-specific error conditions.

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